1. Field of the Invention
The present invention relates generally to an in-line type electron gun used in a color cathode ray tube apparatus, and more particularly to a grid of the electron gun situated in and near the main lens field thereof.
2. Description of the Prior Art
There are many proposals to minimize the influence of spherical aberration against three electron beams. For example, Japanese Laid-Open Publications Nos. 58-103752, 59-215640, and 63-86224 disclose how the effective diameter of a lens situated in and around a main lens field is increased.
In these known systems a focusing grid and a final accelerating grid adjacent thereto are respectively provided with elongated apertures in their opposing end faces, each elongated aperture having a major axis (hereinafter referred to as "major") extending horizontally, and each grid is provided with a field correction metal plate placed along the respective elongated aperture. In addition, three non-circular elongated apertures having majors extending vertically are provided in an in-line arrangement.
These known systems are advantageous in that they can produce a main lens field having a large diameter, but a major disadvantage is that the accurate assembling of an electron gun is difficult to center the components because of the non-circular shape of the apertures produced in the field correction metal plate.
Japanese Laid-Open Patent Publication No. 58-18842 discloses a system in which a focusing grid and a final accelerating grid adjacent thereto are respectively provided with elongated depressions in their opposing end faces, each elongated depression having three circular apertures produced in an in-line arrangement. Under this system, to achieve a desired lens field, the planar side of either depression must be shaped like dumb-bell, thereby making it impossible to use the same shape of components for the two grids. This increases the production cost.